Patent Application
20170050851
The present invention relates to a method for manufacturing carbon quantum dots. More specifically, the present invention is concerned with a method for manufacturing carbon quantum dots via carbonization of self-assembled polymeric nanoparticles.
Quantum dots (QDs) are small nanoparticles having optical and electronic properties different from corresponding macroscopic objects. This phenomenon is prevalent in semiconductors. Indeed, semiconductor quantum dots, such as PbS, CdS and CdSe, have been widely studied as efficient photo-harvesting building blocks for the development of photovoltaic devices and highly active photocatalysts because of their enhanced light-response through size quantization effect. However, potential environmental risks caused by the presence of toxic elements and the imperfect chemical/photo stability of those semiconductor QDs limit their practical applications.
Carbon quantum dots (CQDs) are small carbon nanoparticles, typically less than 10 nm in size, generally with some form of surface passivation. As a class of fluorescent carbon nanomaterials, CQDs generally possess numerous attractive properties, including comparable optical properties to semiconductor quantum dots. Recently, photoluminescent CQDs have indeed been intensely scrutinized due to their low cost, low toxicity, high biocompatibility and good photoluminescence (PL).
Various routes have been developed to synthesize CQDs, such as hydrothermal/microwave carbonization of biomass (e.g., glucose), electrochemical oxidation of graphite, plasma treatment and laser ablation of graphite. Although successful, these synthetic routes present intrinsic limitations which preclude the preparation of CQDs on a large scale. For example, CQDs synthesized by the most popular hydrothermal approach usually require a time-consuming and hardly scalable purification process, such as dialysis to remove reaction residues. Physical approaches, e.g. laser ablation, require a complicated experimental set-up and usually generate small quantities of CQDs. Thus, current synthetic procedures can hardly be implemented on a large scale because they involve high dilutions (dialysis) and extreme experimental conditions (high acidity, high pressure or high voltage). Finally, the resulting CQDs can generally only be stored as dilute colloidal solutions, as they cannot readily be re-dispersed once dried.
In accordance with the present invention, there is provided:
In the appended drawings:
In accordance with the present invention, there is provided a method for manufacturing carbon quantum dots. This method comprises the steps of:
As noted above, the copolymer comprises both insoluble and soluble repeat units. This allows copolymer molecules in the dispersion liquid to self-assemble into self-assembled polymeric nanoparticles with a core/shell structure. More specifically, the insoluble repeat units will tend to congregate into a core to minimize their exposure to the dispersion liquid while the soluble repeat units, not being so driven, will tend to remain as a shell around the core. This will produce a core enriched in insoluble repeat units (i.e. the concentration of insoluble repeat units in the core will be larger than the concentration of insoluble repeat units in the shell) and a shell enriched in soluble repeat units (i.e. the concentration of soluble repeat units in the shell will be larger than the concentration of soluble repeat units in the core). Typically, each nanoparticle comprises several molecules of the copolymer.
The polymeric nanoparticles may comprise a mixture of two or more different copolymers, However, in preferred embodiments, the polymeric nanoparticles comprise a single copolymer.
In embodiments, the copolymer is a block copolymer, the block copolymer comprising at least two different blocks of repeat units: a first block that is insoluble in the dispersion liquid and a second block that is soluble in the dispersion liquid. Again, this allows the copolymer to self-assemble into self-assembled polymeric nanoparticles as the insoluble blocks of multiple copolymer molecules will tend to congregate into a core and the soluble blocks attached to these insoluble blocks will form a shell around the core. In embodiments, the insoluble (first) block is enriched in, comprises all, or consists of, the insoluble repeat units. In embodiments, the soluble (second) block is enriched in, comprises all, or consists of, the soluble repeat units. The block copolymer may comprise more that the above-mentioned two blocks. However, in preferred embodiments, the copolymer consists of the first (insoluble) and second (soluble) blocks only.
The core/shell structure of the polymeric nanoparticles can be seen in
The polymeric nanoparticles self-assemble in a manner similar to micelles. As such, they could be referred to as micelle-like. However, the polymeric nanoparticles are not micelles. Micelles, in particular surfactant micelles, are dynamic. They are characterized by relaxation processes assigned to surfactant exchange and micelle scission/recombination. In contrast, the above polymeric nanoparticles are static and stable once formed. They are not prone to interparticle exchange the way micelles are prone to intermicellar exchange. While the polymeric nanoparticles can, in principle, contain some surfactant, they are not micelles, surfactant micelles, or micelles made of a surfactant; they are nanoparticles made of a copolymer. For clarity, in embodiments of the invention, the dispersion, the nanoparticles, and/or the carbon quantum dots (preferably all of them) are free of surfactants.
In embodiments, the polymeric nanoparticles may further comprise various additives. Non-limiting example of additives include glucose, cellulose, and more generally carbohydrates. In alternative embodiments, the polymeric nanoparticles are free of additives. In fact, in embodiments, the polymeric nanoparticles consist of the copolymer only.
In embodiments, the copolymer comprises carbohydrate repeat units. In preferred embodiments, the carbohydrate repeat units comprise a glucosamine pendant group, which is a good CQD precursor. In more preferred embodiments, the copolymer comprises n-acryloyl-D-glucosamine repeat units.
In embodiments, the copolymer comprises acid repeat units, and/or base repeat units, and/or ethylene oxide repeat units, preferably the copolymer comprises acid repeat units. Acid repeat units are repeat units comprising carboxyls groups (—COOH) Base repeat units are repeat units comprising basic functional groups, such as amine groups [e.g. —NH2 or —NR1R2, wherein R1 and R2 are independently a hydrogen atom, alkyl (preferably C1-8 alkyl) or aryl (preferably phenyl or benzyl), preferably a hydrogen atom or alkyl] and mercapto groups (—SH). Non-limiting example of acid repeat units include acrylic acid, styrene carboxylic acid, itaconic acid, and maleic acid repeat units as well as combinations thereof. Non-limiting example of base repeat units include 2-(N,N-dimethylamino)ethyl methacrylate 2-(N,N-dimethylamino)ethyl acrylate, 2-N-morpholinoethyl methacrylate, 2-diisopropylaminoethyl acrylate, 2-diisopropylaminoethyl methacrylate, N-(3-aminopropyl)acrylamide, N-(3-aminopropyl)methacrylamide, acryloyl-L-Lysine, methacryloyl-L-Lysine, N-(t-BOC-aminopropyl)acrylamide, N-(t-BOC-aminopropyl)methacrylamide, 2-(N,N-dimethylamino)ethyl methacrylate, 2-(N,N-dimethylamino)ethyl acrylate, 2-(tert-butylamino)ethyl acrylate, and 2-(tert-butylamino)ethyl methacrylate repeat units as well as combinations thereof. In more preferred embodiments, the copolymer comprises acrylic acid repeat units.
In embodiments, the copolymer comprises:
Depending on the exact nature of the copolymer used, the produced carbon quantum dots will carry various functional groups. For example, the above acid repeat units will confer acidic functional groups and the above base repeat units will confer basic functional groups to the quantum dots.
In embodiments, the copolymer comprises polymer chains of uniform length and monomer distribution. Without being limited by theory, this is believed to favor a more uniform size for the polymeric nanoparticles and ultimately the carbon quantum dots. Such copolymers can be obtained, among other, by synthesizing the copolymer by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization. RAFT polymerization is one of several known types of controlled radical polymerization. It makes use of a chain transfer agent in the form of a thiocarbonylthio compound (or similar), called a RAFT agent, to afford control over the generated molecular weight and polydispersity during a free-radical polymerization. The RAFT agent, i.e. thiocarbonylthio compounds, such as dithioesters, thiocarbamates, and xanthates, mediate the polymerization via a reversible chain-transfer process. RAFT polymerizations can be performed with conditions to favor low dispersity (molecular weight distribution) and a pre-chosen molecular weight. In embodiments, the RAFT agent is 2-{[(butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid.
In embodiments, the ratio of the number of insoluble repeat units to the number of soluble repeat units in the copolymer is:
In embodiments, the copolymer has a molecular weight (Mn as measured by gel permeation chromatography) of:
In embodiments, the copolymer is present in the dispersion at a concentration of:
In general, increasing the concentration of the copolymer in the dispersion, and/or the above ratio of repeat units, and/or the molecular weight of the copolymer leads to larger nanoparticle sizes, and in turn larger carbon quantum dots. Typically, the carbon quantum dots will have a size of:
Turning now to step a) in more details, in embodiments, this step comprises the steps of:
In step a1), the copolymer is simply dissolved in a solvent. Then, a non-solvent for the insoluble repeat units is added (step a2)). The mixture of solvent and non-solvent forms the dispersion liquid referred to above. The addition of the non-solvent creates conditions favorable to the formation of the nanoparticles. Some agitation (step a3)) allows formation of the nanoparticles, which will be dispersed in the dispersion liquid.
In embodiments, the solvent is ethanol, water, toluene, dichloromethane, chloroform, propanol, methanol, acetone or ethyl acetate, or a mixture thereof. In preferred embodiments, the solvent is a mixture of water and ethanol. In more preferred embodiments, the mixture of water and ethanol has an ethanol/water volume ratio of:
The non-solvent is miscible with the solvent and, as mentioned above, when mixed with the solvent, it forms the dispersion liquid. In embodiments, the non-solvent is water, decanol, nonanol, octanol, heptanol, hexanol, ethyl lactate, diethyl acetamide, octane, nonane, heptane, isopare, xylene, durene, dichlorobenzene, or a mixture thereof. In preferred embodiments, the non-solvent is heptanol, preferably n-heptanol.
It should of course be understood that while the above lists of examples for the solvent and non-solvent overlap, in any given embodiment of the invention, the solvent is different from the non-solvent.
In embodiments, the mixture obtained in step a2) has a solvent:non-solvent volume ratio of:
In embodiments, in step a3), the mixture is agitated by sonication, by mechanical stirring (such as with a stirrer and a blade or with a magnetic stir-bar), by ball-milling, by homogenization (using a rotor stator assembly), and/or by microfluidization, preferably by sonication.
Turning now to step b) in more details, in embodiments, the carbonization of the nanoparticle cores in step b) is effected by heating the dispersion at a temperature equal to, or higher than, a carbonization temperature of the insoluble repeat units. In preferred embodiments, in step b), the dispersion is heated at said temperature and then refluxed at said temperature. Of course, the specific carbonization temperature of the insoluble repeat units will depend on their nature. In embodiments, the dispersion is heated and then refluxed at a temperature ranging of:
Preferably, the dispersion is heated and then refluxed at a temperature of about 170° C. In embodiments, the reflux lasts:
In embodiments, the method further comprises the step c) of isolating the carbon quantum dots from the dispersion liquid. In preferred embodiments, in step c), the solvent and the non-solvent are evaporated.
In embodiments, the method further comprises the step d) of dispersing the carbon quantum dots [after they have been isolated in step c)] in a liquid. In preferred embodiments, in step d), the carbon quantum dots are agitated in the liquid. In preferred embodiments, in step d), the carbon quantum dots are agitated by sonication, by mechanical stirring (such as with a stirrer and a blade or with a magnetic stir-bar), by ball-milling, by homogenization (using a rotor stator assembly) and/or by microfluidization. In embodiments, the liquid in step d) is water or a polar solvent, preferably an organic polar solvent (such as an alcohol). In preferred embodiments, the liquid is water.
In embodiments, the method further comprises the step e), which may be carried out at any time after step c), of recycling the solvent and/or the non-solvent.
One or more embodiments of the above method may have one or more for the following advantages:
In fact, the method of the invention allows access to high-quality, easily dispersible carbon quantum dots (CQDs). This is essential to fully exploit the desirable properties of carbon quantum dots.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context.
The terms “comprising”, “having”, “including”, and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to”) unless otherwise noted.
Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All subsets of values within the ranges are also incorporated into the specification as if they were individually recited herein.
All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.
The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed.
No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Herein, the term “about” has its ordinary meaning. In embodiments, it may mean plus or minus 10% or plus or minus 5% of the numerical value qualified.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.
The present invention is illustrated in further details by the following non-limiting examples.
We report below an efficient approach to synthesize high-quality dispersible CQDs using self-assembled polymeric nanoparticles.
More specifically, copolymers based on N-acryloyl-D-glucosamine and acrylic acid prepared by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization were self-assembled into polymeric nanoparticles (herein also called nanoreactors). After a facile graphitization process (170 00, atmospheric pressure), each resulting CQD was a 1:1 copy of the nanoreactor template. The high-quality CQDs (quantum yield≈22%) with tunable sizes (2-5 nm) were decorated by carboxylic acid moieties and could be spontaneously re-dispersed in water and polar organic solvents.
To demonstrate the versatility of this approach, CQDs hybridized TiO2 nanoparticles with enhanced photocatalytic activity under visible-light have been prepared.
Our templating approach is based on the use of self-assembled polymeric nanoparticles which are not prone to interparticle exchange. As shown in
Subsequently, 1-heptanol is introduced to the polymer solution and sonicated to form a light yellow cloudy solution (b). Since PAGA is insoluble in heptanol whereas PAA is soluble, stable polymeric nanoparticles self-assembled upon the addition of 1-heptanol (
This solution was then heated to ˜170° C. and refluxed for ˜40 minutes under N2 until a stable brown color was reached. During this thermal treatment, water evaporated, and carbonization was triggered upon intermolecular dehydration of the AGA units to form CQDs (c). Heptanol was then distilled using a simple vacuum distillation in order to be recycled for future uses and the as-prepared dry CQDs can be easily redispersed in water or other polarity solvents (e.g. alcohol) by low power ultrasound (10 minutes sonicator bath, 90 W).
By simply varying the amount of the polymer used for carbonization, CQDs with controllable particle sizes were easily achieved.
The decrease of the intensity of the —OH band is due to the dehydration of the glucose groups of AGA units during the carbonization process. The persistence of the ˜3100-3600 cm−1 of CQDs is mainly attributed to the —COOH groups of AA units, which render the surface of the CQDs hydrophilic and make them self-dispersible in water. Additionally, the AA units acting as ligand molecules on CQDs surface could generate more defect sites on the CQD surface, thus enhancing their optical performance (Sun et al., J. Am. Chem. Soc. 2006, 128, 7756; Kwon and Rhee, Chem. Commun. 2012, 48, 5256).
Under white light, the smoky yellow tinge of the CQD solution becomes more pronounced as the particle size increases from ˜2.1 nm to ˜3.6 nm (
The typical UV-Visible (UV-Vis) absorption spectrum of the CQDs (
The optical property of the CQDs was further studied using the PL spectroscopy.
Based on PL measurement, the quantum yield of the CQDs was calculated to be 21.8% (see
More specifically, according to literature (Kwon and Rhee, Chem. Commun. 2012, 48, 5256), the quantum yield (QY, φ) of CQDs was calculated by using quinine sulfate (QS) as the standard. To calculate the QY, five concentrations of each sample were prepared with absorbance less than 0.1 at 340 nm. QS (literature φ=0.54) was dissolved in 0.1 M of H2SO4 (refractive index (η)=1.33), and the CQDs were dispersed in absolute ethanol (η=1.36). Their PL spectra were recorded at λex=340 nm. Then, PL intensities (λex=340) and the absorbance (at 340 nm) of the CQD samples and the QS references were compared. The PL-Absorbance data were plotted (
The QY of the CQDs was calculated using the following equation:
φC=φST(mC/mST) (ηc2/ηST2)
where φc is the QY, m is the slope, η is the refractive index of the solvent, ST is the standard and C is the sample. The QY for CQD was thus calculated to be ˜21.8%.
The particle size of the CQDs can be tuned simply by varying the polymer structure.
Three polymers with AGA/AA ratio of 1.5/1, 1/4.6 and 1/26 were synthesized. CQDs were subsequently synthesized using 240 mg of the three polymers and characterized by TEM. The results are reported in Table 1.
aDetermined by 1H nuclear magnetic resonance (1H-NMR) spectroscopy (FIG. 9).
bMolecular weight determined by gel permeation chromatography (GPC).
cCQD size determined by TEM.
As shown in
The samples showed typical size-dependent luminescence features under 365 nm UV light (insets of
The origin of this size control is ascribed to the adjustment of the glucose content in the nanoreactor. Increasing the amount of the polymer or the number of AGA units in the polymer chain can lead to a glucose enrichment in the nanoreactor, and thus results in a larger CQD particle size. Once again, the dry CQDs were readily re-dispersed in water under ultrasound.
To demonstrate scalability, 8.74 g of P(AGA)(AA) polymer were transformed in ˜273 mg of dried CQDs which were readily re-dispersed in water.
By contrast, dry CQDs synthesized by a hydrothermal approach were not fully dispersed in water (see
Coupling with TiO2
To testify the performance of the polymeric method synthesized CQDs, the CQDs (Dmean=˜3.0 nm, herein) were coupled with TiO2 nanoparticles to form TiO2/CQD nanohybrid catalyst for the photodegradation of methylene blue (MB) under visible-light (λ>420 nm).
Due to the presence of negatively charged carboxyl groups (—COO−) on CQD surface in water (pH=6-7), the CQDs were efficiently adsorbed on the surface of TiO2, whose surface was slightly positively charged, through an electrostatic interaction.
Through measuring the intensity of the UV-Vis absorption peak of MB solution, the degradation process could be monitored (Zhang et al., Sol. Energy Mater. Sol. Cells 2002, 73, 287)—see
D-glucosamine hydrochloride, acryloyl chloride, acrylic acid (AA), 4,4-azobis (4-cyanovaleric acid) (ABV) and TiO2 nanoparticles were purchased from Sigma-Aldrich. Potassium carbonate (K2CO3), sodium nitrite (NaNO2), methylene blue, hydrochloric acid (HCl), absolute ethanol and 1-heptanol were purchased from Fisher Scientific. RAFT agent of 2-{[(butylsulfanyl)carbonothioyl]sulfanyllpropanoic acid was synthesized as reported by Ferguson et al, in Macromolecules 2005, 38, 2191. The AA was purified using vacuum distillation before using. Other chemicals were used without further purification. Water was Nanopure grade (18.2 MΩ·cm at 25° C.).
Typically, 8.06 g of D-Glucosamine hydrochloride and 0.14 g of NaNO2 were dissolved in 20 mL of K2CO3 aqueous solution (2 M). This solution was purged with nitrogen and cooled to ˜0° C. in an ice bath under vigorously stirring. 4.0 g of Acryloyl chloride was added drop wise over 1 hr. The reaction solution was kept below 5° C. for ˜3 additional hours, while stirring was maintained. After warming to room temperature and stirred for one day, the dispersion was poured into 200 mL of cold absolute ethanol, and refrigerated overnight. After the precipitated salts were filtered off, the resulting solution was dried under vacuum and the product was purified by re-crystallization with methanol (75%) to achieve white powder. The product yield was ˜47%. 1H-NMR (D2O, 300 MHz, δ): 6.37-6.08 (m, 2H), 5.74 (dd, J=9.8, 1.8 Hz, 1H), 5.17 (d, J =3.5 Hz, 1H), 4.08-3.26 (m, 8H).
i) Synthesis of P(AGA)(AA): Typically, a mixture of 120 mg of RAFT agent, 100 mg of AGA, 60 mg of AA and 10 mg of ABV (mole ratio of AGA: AA=1:2) were dissolved in 4 mL of degassed ethanol/water (volume ratio 2:1) solution. This solution was heated to 70° C. under stirring with the protection of N2 for 3 hrs to complete the polymerization. The resulting polymer was purified/recovered by precipitation in cold ethanol and dried under vacuum. Yield: 200.5 mg (71.6%). This recipe led to the polymer with AGA/AA ratio of 1.5/1.
ii) Synthesis of CQDs: Typically, 56 mg of the P(AGA)(M) was dissolved in 4 mL of the degassed ethanol/water (volume ratio 2:1). Subsequently 7 mL of heptanol was injected to the polymer solution and sonicated to form a homogenous light yellow cloudy solution. This solution was then heated to 170° C. quickly and refluxed for 40 min with vigorous stirring under N2 flow until a stable light brown color was achieved. Afterwards, heptanol was removed/recycled by evaporation-condensation under vacuum. The brown residue was cooled down to room temperature and re-dispersed in water by sonication. The turbid light brown aqueous solution was then centrifuged at 3500 rpm for 10 min and the flocculate deposit was discarded. The clear yellow supernatant was collected and filtered using a cellulose syringe filter with pore size of 0.22 μm. The received filtration containing CQDs was then used for characterization and catalytic application.
The amount of RAFT agent and monomers (AGA-FAA) can be magnified to scale up the polymer quantity for the CQDs synthesis. On the other side, under the same reaction and purification conditions, the feeding monomer mole ratio of AGA and AA for polymerization can be adjusted to be 1:17 and 1:30 to achieve the polymers with AGA/AA ratio of 1/4.6 and 1/26, respectively. The polymers with different structures were used for CQDs synthesis as mentioned above.
To 20 mL of CQD aqueous solution 30 mg of P-25 commercial TiO2 powder was added. The resulting dispersion was sonicated in sonicator bath for 5 minutes and then heated at 60-70° C. under stirring until the water evaporated completely. The resulting light-yellow powder was then transferred in conventional oven and heated at 300° C. in air for 30 min and cooled to room temperature automatically.
30 mg of catalyst samples (TiO2 and TiO2/CQDs) were dispersed in 15 mL of 80 mg/L MB aqueous solution under vigorous stirring in darkness for 6 h to reach an equilibrium adsorption for MB. The solution was centrifuged and the catalyst was washed with a small amount of water and re-dispersed in 10 mL of fresh 8 mg/L MB solution. The dispersion was then irradiated at room temperature using a solar light simulator (Sciencetech Inc., SS0.5KW.) with a cutoff filter (λ>420 nm). The average light intensity was ˜70 mW/cm2. At regular intervals, aliquots were removed and analyzed by UV-Vis spectroscopy.
i) FT-IR. The polymer and CQDs were analyzed using a Nicolet 6700 FT-IR spectroscopy equipped with an ATR accessory. ii) TEM. The CQDs was imaged using JEOS-2100F TEM (École Polytechnique de Montréal, Montréal, Canada). iv) 1H-NMR. Proton nuclear magnetic resonance spectra of the monomer and copolymer were recorded with a Bruker 300 (300 MHz) instrument using Deuterium oxide (D2O) as solvent. v) UV-Vis spectroscopy. UV-Vis absorption spectra were collected using a Varian Cary 100Bio spectrometer. All measurements were done at room temperature. vi) PL. PL property and lifetime of the samples were measured using a Varian Cary Eclipse fluorescence spectrophotometer. vii) GPC. Molecular weight of the polymers was determined using a GPC with water as the mobile phase and equipped with a Wyatt Dawn 18 angle light scattering detector and a Dawn DSP refractometer. viii) DLS. Malvern Zetasizer Nano S-90 was used to measure the size of polymer solution.
The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety. These documents include, but are not limited to, the following:
This application claims benefit, under 35 U.S.C. §119(e), of U.S. provisional application Ser. No. 62/206,453, filed on Aug. 18, 2015.
| Number | Date | Country | |
|---|---|---|---|
| 62206453 | Aug 2015 | US |
